1. Field of the Invention
The present invention relates to a fixing device configured to be installed in an image forming apparatus to fix a toner image on a recording medium, and further relates to an image forming apparatus including the fixing device.
2. Description of the Related Art
In well-known electrophotographic image forming apparatuses, such as a printer, a facsimile machine, a copier, or a multifunction peripheral (MFP) having at least two of these functions, typically, there is widely used a configuration that an electrostatic latent image is formed on the surface of an image carrier such as an photosensitive element; the electrostatic latent image is developed into a visible image with toner which is developer; the developed toner image is transferred onto a recording medium such as a recording sheet, and the recording medium carries the image (the unfixed toner image) thereon; and a fixing device applies heat and pressure to the unfixed toner image on the recording medium thereby fixing the toner image on the recording medium. In the fixing device of this electrophotographic image forming apparatus, the recording medium carrying the unfixed toner image thereon is held in a fixing nip formed between rollers or belts opposed to each other or a combination of these, and heated while being subjected to pressure, which makes the unfixed toner image fixed on the recording medium.
Among such fixing devices, an electromagnetic induction heat-fixing device using a coil which generates a magnetic flux or magnetic field lines is known and already widely used. This electromagnetic induction heat-fixing device includes, for example, a fixing member such as a fixing roller provided with a thin-walled fixing sleeve having a heating layer on the outer circumference of a heat-insulating elastic layer, a pressure member such as a pressure roller which presses against the fixing member thereby forming a fixing nip, and an electromagnetic induction heating member which is placed close to or to be opposed to the outer circumferential surface of the fixing member and heats the fixing member by means of electromagnetic induction. The electromagnetic induction heating member is composed of a (exciting) coil, a core for covering the coil, a coil guide which holds the coil and causes the coil to be opposed to the fixing member, and the like. By passing a high-frequency alternating current through the coil of the electromagnetic induction heating member, a magnetic flux which is alternately switched in two ways, i.e., an alternating magnetic field is formed around the heating layer that the fixing sleeve or the like provided in the fixing member has, and an eddy current is generated in the heating layer by the alternating magnetic field, and the heating layer and, eventually, the fixing sleeve set in the fixing member is heated by Joule heat generated by electric resistance of the heating layer to the eddy current. By means of heat from the fixing member provided with the fixing sleeve heated in this way, toner on a recording medium conveyed to the fixing nip is fused, and by means of pressure from the pressure roller pressing against the fixing member at the fixing nip, the fused toner is fixed on the recording medium as a semi-permanent image.
To form the fixing nip and to ensure efficient heat generation, the fixing sleeve of the fixing member in this kind of electromagnetic induction heating method is formed into a thin-walled structure. Therefore, for example, if the fixing sleeve is a defective part and has some scratches from the beginning, or if heating runaway occurs, there is a problem that the fixing sleeve is easily damaged. Besides, the heat-insulating elastic layer located on the side of the inner circumference than the fixing sleeve having the heating layer is also subjected to the pressure from the pressure member such as a pressure roller via the fixing sleeve or the like for a long time to form the fixing nip; therefore, the heat-insulating elastic layer may be damaged with time.
To cope with the damage to a component of such a fixing device, for example, Japanese Patent Application Laid-open No. 2007-328159 discloses a conventional technology for detecting damage to a fixing belt. In the conventional technology, an energization prohibiting means is placed at the position opposed to a coil, which is an electromagnetic induction heating member, across an endless belt which is the fixing belt. The energization prohibiting means has an antenna which generates a voltage or electric current from a magnetic flux from the coil. Regardless of temperature of the endless belt, if the voltage or electric current generated in the antenna exceeds a predetermined amount, the energization prohibiting means prohibits energization of the coil. If the endless belt is not damaged, the antenna in the energization prohibiting means does not detect any voltage or electric current because the antenna is shielded from the coil by the endless belt. However, if the endless belt is damaged, the antenna is opposed to the coil because there is no endless belt in the damaged portion, and a voltage or electric current is generated in the antenna due to a magnetic flux from the coil; the damage to the endless belt is detected with this.
Using the conventional technology, breakage or damage of the endless belt can be detected by the action of the antenna of the energization prohibiting means. However, it is not possible to detect damage to a fixing roller which supports the endless belt. From the experience of the present applicant, this kind of fixing roller can be suddenly broken with time due to pressure contact with a pressure roller. That is, the conventional belt damage detection cannot resolve the problem of damage to the fixing roller.
Further, in the fixing device according to the conventional technology, the energization prohibiting means prohibits energization of the coil, so a fixing process and, eventually, image forming operation cannot be performed. However, when the fixing roller is damaged, in most cases, the damage begins in an end portion of the fixing roller, so the fixing device may be able to be still used depending on the size or type of a recording medium subjected to a fixing process. In this case, it is user-friendly or beneficial to a user, i.e., convenient for a user if an image forming apparatus can be used depending on a recording-medium feeding condition, such as the size or type of a recording medium, until the fixing roller is replaced.